1. Field of the Invention
This invention relates in general to certain new and useful improvements in a device and method for a series port connection of a plurality of terminals to a master processor, and more particularly, to a device and method which enable a plurality of terminals to be connected in parallel across a data input bus and a data return bus from a central processor and where each of the terminals are connected in series with respect to the master central processor.
2. Brief Description of the Prior Art
Various circuit arrangements for transferring analog information between a central console and a plurality of terminals addressing that central console or central station are well known and, in general, utilize a conventional time sharing arrangement. These systems require multiple bus lines for transmitting the terminal address. Exemplary of such prior art analog time sharing systems for analog data transmission is that system described in U.S. Pat. No. 3,633,165 to Gilbert.
In the case of digital data transmission as for example, between a central computer or central microprocessor and a plurality of terminals, the connection of the terminals to the central computer may become a more complex arrangement. With the advent of small portable and so-called "desk top computers" with large memory capabilities, there has been a recent interest in utilizing the small desk top computer as a central processor or main computer station with a plurality of terminals accessing the central station desk top computer. However, most of the conventionally available desk-top or small portable computers only have a very limited number of serial ports and thus, the number of computer terminals which may be connected to and access that central computer station are limited by the number of serial ports on the computer at that central station.
In recent years, there have been several available rather sophisticated solid state switching devices to enable the use of time division for information transmission. In this way, sampling techniques are used to enable a large number of users to communicate over a single line or a single bus while only selected interconnections between a central station and individual terminals are established.
These time division communication systems are relatively complex and expensive and are not adaptable for many uses. Moreover, they are not commonly used on smaller computers. In addition, they require specialized hardware in both the computer and the terminal. Exemplary of such prior art systems is U.S. Pat. No. 3,916,108 to Schwartz.
Telemetry systems which multiplex data from a plurality of sources onto a common bus are described in U.S. Pat. No. 4,386,426 to Pugh. Each of the terminals, which are considered to be remote stations, are connected to the data buses individually and are series connected relative to one another. Here again, the system in the Pugh patent also operates on a time division arrangement.
Another arrangement for collecting information at and distributing information from a master station from a number of terminal stations is disclosed in U.S. Pat. No. 3,824,545 to Brenner et al. In the Brenner et al system, a plurality of terminals are connected to a master station over a single transmission line and a plurality of collecting lines. Each of the terminals utilizes an ordered mutually time division arrangement for transmission of the information. Each terminal station is actuated after the preceding terminal station and before the next subsequent station. For this purpose, each terminal station is provided with a delay unit actuated by the trailing edge of a pulse supplied by the preceeding terminal station.
U.S. Pat. No. 3,217,177 to Walker discloses logic circuits which use diode gates on the outputs to a single transmission line. Each diode gate drives an emitter-follower arrangement and each gate is biased to draw low current so as to preclude the loading of any other gate signal source to thereby eliminate large reflections on the transmission line.
U.S. Pat. No. 2,708,220 to Levy also discloses a multi-channel communications system in which exchanges, such as telephone exchanges, may be connected to one another by several channels. Communication again occurs on a time-division pulse arrangement. This patent also discloses signal direction controlling diodes in the output lines.
Heretofore, there has not been any available low cost system which enables a plurality of relatively inexpensive computer terminals to access a relatively inexpensive small computer through a single serial access port without the necessity of engaging in complex time-division arrangements and the attendant circuitry.